Magnesium based coating for the sacrificial protection of metals

ABSTRACT

A METHOD IS PROVIDED FOR IMPROVING THE CORROSION RESISTANCE OF MATERIAL SUBSTRATES. THE METHOD IS PARTICULARLY APPLICABLE TO THE PROTECTION OF METAL SUBSTRATES, SUCH AS FERROUS METAL ARTICLES, AMONG OTHER METALS, AND, IN PARTICULAR, TO THE PROTECTION OF LOW ALLOY STEEL GAS TURBINE ENGINE COMPONENTS FOR AIRCRAFT IN WHICH THE SURFACE IS PROVIDED WITH AN ADHERENT PROTECTIVE LAYER OF A SACRIFICAL COATING RICH IN MAGNESIUM AND ALSO CONTAINING SILICON, OXYGEN AND OPTIONALLY IRON. THE COATING IS CHARACTERIZED BY THE PRESENCE OF MAGNESIUM SILICIDE AS A MAJOR ACTIVE CONSTITUENT.

United States Patent O 3,748,172 MAGNESIUM-BASED COATING FOR THE SACRIFICIAL PROTECTION OF METALS Kenneth K. Speirs, Universal City, and Martin Weinstein and Roy L. Blize, San Antonio, Tex., assignors to The Chromalloy American Corporation No Drawing. Filed May 17, 1971, Ser. No. 144,225 Int. Cl. C23f 7/00, 17/00; C23c 9/02 US. Cl. 117-135.]. 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the protection of material substrates, particularly the protection of metal substrates, such as ferrous and non-ferrous metals, from corrosion in highly saline and/or marine atmospheres.

FIELD OF THE INVENTION Jet and gas turbine engine compressor components, for example, discs and blades, are subject to corrosion in highly saline atmosphere at the air intake end of the engine and also to direct impact of abrasive particulate matter, such as coral dust. Additionally, compressor discs and blades among other components are subjected to tremendous mechanical stresses from centrifugal forces, thermal shock, vibration and other sources of stresses. Thus, corrosion can accelerate catastrophic failure, since pits and other corrosion defects can act as stress raisers.

High strength ferrous alloys are employed in the construction of compressor discs, spacers, blades and other aircraft engine components (eg Society of Automotive Engineers alloys designation AMS 6304, SAE 4340, AMS 5508, AMS 5616, and others) but, because of their low resistance to saline corrosion, they are generally subjected to a protective surface treatment. One in particular, is the provision of an aluminum-base diffusion coating on the ferrous substrate by pack-aluminizing at coating temperatures ranging up to about 1000" F. (538 C.) and preferably not higher so as to avoid undesired crystallographic or metallurgical changes inthe substrate during coating, which might have an adverse or undesired effect on the mechanical properties of the parts. While such coatings provide advantageous oxidation and erosion resistance and minimize the production of pulverous corrosion products on alloys, such as AMS 5616 12% chromium steel), they are not sufliciently anodic with respect to low alloy steel substrates, such as AMS 6304 (less than 3% chromium and less than 1% nickel), to offer the desired sacrificial or anodic protection thereof against saline corrosion.

A surface treatment involving a magnesium-based coating has now been discovered which provides optimum resistance to corrosion for prolonged periods of time. As far 3,748,172 Patented July 24, 1973 as is known, this novel treatment for protecting metal substrates was not known prior to this invention.

OBJECTS OF THE INVENTION It is thus the object of this invention to provide a sacrificial coating for the protection of material substrates, such as metal substrates.

Another object is to provide a method for further enhancing the corrosion resistance of ferrous and non-ferrous metal substrates, particularly low alloy steel substrates.

A still further object is to provide an improved substantially sacrificial coating system for protecting ferrous and non-ferrous metal surfaces.

Another object is to provide an improved sacrificial coating system for protecting certain anodically active metals, such as aluminum and aluminum alloys.

A further object is to provide a method of protecting materials of construction which are subject to galvanic corrosion at their contacting substrates by interposing between said materials a sacrificial coating that preferentially corrodes anodically relative to both substrates.

These and other objects will more clearly appear when taken in conjunction with the following description and the appended claims.

STATEMENT OF THE INVENTION This invention is based on the discovery that a highly adherent composite sacrificial coating rich in magnesium as an essential ingredient provides unexpected improved galvanic protection for a wide range of substrate materials, such as ferrous and non-ferrous metal substrates, for example, low alloy steel, aluminum, aluminum alloys, titanium, titanium alloys and other metals. Normally, in the case of stainless steels, galvanic protection has been achieved with a thermally diffused aluminum coating based on an iron-aluminum intermetallic. However, the required electromotive force difference between the aluminum coating and low alloy steel substrates is not always suflicient for severe corrosion environmental conditions. It has been found that the behavior of a protective coating based on thermally diffused aluminum or noble coating of nickel can be detrimental in that localized breakdown of the coating can often lead to aggravated sub-surface pitting attack.

While it is apparent that pure magnesium would provide excellent sacrificial protection for steels, a main disadvantage of pure magnesium is that it tends to react spontaneously with water and, as a consequence, such coatings would not have sufiicient environmental stability. Moreover, it is practically impossible to transfer magnesium onto steel by the pack cementation process because of its insolubility in iron. The invention overcomes the foregoing difiiculties and provides a magnesium-rich coating having excellent sacrificial properties.

Thus, stating it broadly, one embodiment of the invention resides in providing a substrate to be coated, applying a silicon-containing coating of finite thickness to the substrate, e.g. a coating of sodium silicate, subjecting the coated substrate to a pack diffusion process by embedding the substrate in a pack comprising a magnesium-containing constituent, preferably in particulate form, and a small but effective amount of a halide energizer, subjecting the pack assembly to a temperature sufiicient to cause transfer of magnesium to the substrate,

for example, at a temperature of about 700 F. (400 C.) to 1000 F. (538 C.) in a retort, and then removing the treated substrate from the pack, such that an adherent sacrificial coating is obtained rich in magnesium and also containing oxygen, silicon and possibly iron, where the substrate is a ferrous metal. In a preferred embodiment, after the application of the magnesium coating, a silicate overcoat (e.g. sodium silicate) is applied to the surface thereof and thereafter thermally cured. An analysis of the coating indicates that a major portion of the magnesium is present as a magnesium silicide. The pack into which the part is embedded preferably contains an inert powder diluent, such as alumina.

In one embodiment of the invention as applied to a bare steel specimen, the specimen was coated with sodium silicate and the coated specimen placed in a cementation pack containing about 20% magnesium, 78% aluminum oxide and 2% aluminum chloride. The pack, which was confined in a retort, was placed in a furnace and heated to 900 F. for about 16 hours. A coating was produced as a reaction product of the silicon-containing coating with the active ingredients in the pack, the coating being enriched in magnesium, a major portion of which is in the form of magnesium silicide, the coating also containing oxygen and iron.

DETAILS OF THE INVENTION In producing the coating, three steps may be employed, to wit: preparing the substrate to receive the coating, applying the silicon-containing layer (e.g. sodium, potassium or lithium silicate) and then thermally treating the coated substrate in a magnesium pack. As an optional fourth step, an overcoat of silicate or other conversion coating is preferably applied.

In preparing a ferrous metal substrate to receive the silicon-containing layer, the surface is preferably first cleaned. A preferred method is to hone it with finely divided aluminum oxide as this yields the best surface consistent with assuring good fatigue properties. While chemical cleaning can be employed, it is not preferred since it can have some effect on lowering the fatigue properties. The silicon-containing layer, e.g. sodium silicate, should be applied as uniformly as possible using, for example, a sodium silicate or potassium silicate solution of predetermined concentration. The part is immersed in the silicate solution and the excess removed by first draining it oil and then blowing air over the surface. After drying the layer, it is subjected to a curing cycle at, for example, 400 F. (205 C.) to expel excess moisture. The part is allowed to cure and an additional layer applied in the same manner. Additional coats may be applied, depending upon the thickness to be achieved.

As stated above, the pack may comprise a mixture of a magnesium-containing constituent in particulate form (e.g. from about 30 to 325 mesh) and an inert powder diluent, such as refractory oxide, e.g. alumina, plus a small but effective amount of a halide energizer. A pack composition which has been found particularly useful is one containing 40% by weight of nominally 100 mesh magnesium and 60% by Weight of aluminum oxide normally about 200 mesh. To the mixture is added about 2% by weight of, for example, ammonium chloride. The pack is placed in atwo-part retort, the part embedded in it and the retort lid secured to the retort flanges with layers of aluminum inserted therebetween as a gasket and the lid then bolted to the flange.

The part is subjected to pack diffusion for about 16 hours in an oven maintained at an elevated temperature conducive to effect transfer of the magnesium to the substrate, e.g. between about 800 F. to 900 F. (about 427 C. to 482 C.). The retort is then removed from the oven and allowed to air cool, any excess powder adhering to the part being removed by blowing off with air or by immersing the part in a solution of water and silicate in an ultrasonic cleaning tank. The part i then dried and, as

an optional step, a silicate overcoat applied. In one embodiment, the overcoat is applied with a nominal 1% by volume silicate solution of, for example, sodium silicate (diluted from 41.5 Baum solution), by immersion in the silicate solution, the excess solution being thereafter removed from the part followed by drying and then curing at about 400 F. (205 C.). Normally, three layers of silicate overcoat are applied in this manner. The foregoing silicate solution amounts to about 0.3% by Weight of SiO equivalent. Broadly speaking, the silicate solution, (e.g. sodium silicate, potassium silicate, lithium silicate and ethyl silicate) may range by weight from 0.1% to 17.5% SiO equivalent. At the higher end of the range, the silicate solution is preferably applied by spraying.

The foregoing coating provides markedly improved protection of the substrate when subjected to the salt spray test, complete protection having been observed for times up to 500 hours in the salt spray cabinet. This test is based on the procedure outlined in ASTM B 117-64.

The ASTM salt spray test (Designation B 117-64) employed in testing the resistance to corrosion of the various coating systems disclosed herein comprises a fog chamber, a salt solution reservoir, a supply of suitably conditioned compressed air, one or more fog nozzles, specimen supports, provisions for heating the chamber and control means. The specimens are supported or suspended between 15 and 3-0 degrees from the vertical (out of contact with each other) and preferably parallel to the principal direction of horizontal flow of fog through the chamber. The salt solution is made up ,of 5:1 part of salt to parts of distilled Water containing not more than 200 p.p.m., of total solids. The condensed fog should have a pH of about 6.5 to 7.2. The temperature within the chamber is maintained at 95 F. plus 2 or minus 3 F. For the specimens in this case, the salt spray testing is carried out for a period stated herein, precautions being taken to avoid dripping of condensed solution from one specimen to another.

In using the test to evaluate the quality of the sacrificial coating, specimens comprising V2 inch cylinder or 1 inch strip of the substrate are employed. In the case of the strip, a section of the coating is abraded from the specimen to be tested. In the case of the cylindrical specimen, one edge is bevelled by abrasion on a belt or grinding wheel to expose the substrate. The specimens with the partially exposed substrate are then subjected to the aforementioned ASTM salt spray test. The sacrificial coating gave excellent results after 500 hours of testing as evidenced by the complete freedom of substrate deterioration.

Very good protection has even been observed after exposure of the coated part at 900 F. (482 C.) followed by the salt spray test. However, the coating is more efiective at temperatures up to 850 F. (455 C.). Excellent salt spray protection has been obtained after exposure at temperatures up to about 800 F. (427 C.).

The coating thickness has been found to vary between 0.2 to 1 mil (0.0002 to 0.001 inch) and is a function of the silicate thickness applied. Very good protection occurs at a nominal thickness of about 0.5 mil. Metallographically, the coating has a dark structure which is free of microcracks. 7

Fatigue tests have indicated that the coating does not adversely affect the endurance limit of the substrate metal. Thus, high cycle fatigue tests using rotating beam specimens have substantiated that the bare and coated specimens have the same endurance limit.

EMF measurements in a 3% sodium chloride solution with a calomel reference electrode showed 1.2 volts for the magnesium-based coating as compared to 0.56 volt for Aluminum Alloy No. 2016, 0.45 volt for low alloy steel designated as AMS 6304 and 0.11 volt for stainless steel bearing the designation AMS 5616.

A chemical profile of the coating as determined by the electron microprobe has established that the coating can have levels of approximately 50% Mg, 20% Si, 20% oxygen and varying levels of iron on a steel substrate. An analysis has indicated that a major portion of the magnesium is present in the form of magnesium silicide. This has been confirmed by X-ray diffraction.

The coating is apparently a reaction product resulting from the pack cementation process. In some cases, the iron compositional profile is such that an iron solid solution occurs at the substrate coating interface, indicating that diffusion of iron from the steel substrate into the coating has occurred. The magnesium level, according to the electron microprobe, has a peak associated with the silicon peak and both are at lower levels at the outer surface of the coating.

The exceptionally high electrochemical potential of the coating (1.2 volts as compared to about 1.3 volts for pure magnesium) and the analyzed 50% magnesium in the coating indicates that the magnesium in the coating exhibits a high positive deviation from ideality. This behavior is very unique in this type of coating. Generally, the coating may contain about 20% to 50 or 60% by weight of magnesium, and the balance essentially silicon, oxygen and some constituents derived from the substrate.

As illustrative of the invention, the following example is given.

EXAMPLE 1 A compressor disc of AMS 6304 low alloy steel and sample coupons thereof are cleaned by honing with finely divided aluminum oxide powder. The disc is then immersed in a heated potassium silicate solution of 10% by volume concentration derived from a 30.2 Baum solution. The 10% by volume concentration corresponds to about 2.1% by weight of Si equivalent. The solution is maintained in the temperature range of about 140 F. (60 C.) to 160 F. (70 C.), and disc and coupon alternately immersed and raised above the solution to enable the liquid to drain off. Any excess liquid is blown off the surface with compressed air after which the disc and coupons are again immersed in the solution, followed by draining and drying. The foregoing is repeated seven times to provide seven layers of the silicate. The coated parts are then cured in an oven for 30 minutes at about 400 F. (205 C.). After the foregoing treatment, the parts are embedded in a pack comprising 50% by weight of 30 mesh magnesium (US. Standard) and about 50% by weight of 30 mesh aluminum oxide. The pack is sealed in a retort which is then placed in an oven and heated for 30 hours at 800 F. (427 C.). After completion of the coating, the parts are removed from the powder and cleaned by washing in a hot solution of potassium silicate maintained at a temperature of about 140 F. (60 C.) to 160 F. (71 C.).

The coupons treated together with the discs in the foregoing manner provided a coating which exhibited excellent sacrificial properties when subjected to the salt spray test.

The corrosion resistant properties were even further enhanced by applying an overcoat of potassium silicate from a solution containing-about 25% by volume of 30.2 Baum potassium silicate (about 5.2% by weight of SiO equivalent), the overcoat being applied in three cycles by dipping, draining, drying and curing until the desired thickness is obtained or by spraying and curing. Where the silicate is applied by dipping, the solution concentration may range from about 0.2% to 5% SiO equivalent. Where the coating is sprayed, the solution concentration may range by weight from about 5% to 17.5% SiO equivalent. Broadly, the solutions may range from about 0.2% to 17.5% SiO equivalent.

EXAMPLE 2 A group of compressor blades of AMS 5616 stainless steel is cleaned by honing with aluminum oxide powder.

The blades are then racked and the rack immersed in a tank containing an aqueous solution of 10 to 25% by volume of 41.5 Baum sodium silicate (about 2.9% to 7.3% by weight of SiO equivalent). The temperature of the bath is maintained at about F. (26 C.) to 100 F. (38 C). After the blades have been immersed to completely wet the surface, the rack is removed and excess liquid allowed to drain off and the blades dried by blowing with air. The blades are then recycled in the solution and dried by blowing off with air for a total of three applications. The dried blades are then subjected to a curing cycle at 400 F. (205 C.) for 15 minutes. This constitutes one silicate application. The blades are then allowed to cool and two additional applications of silicate made in the same manner. The foregoing silicate cycles provide a silicate thickness of about 0.4 mil.

After the blades have been silicate treated, they are embedded in a pack containing about 50% by weight of 50 mesh magnesium powder (US. Standard), about 50% by weight of alumina (200 mesh) and ammonium iodide added in amounts of about 2% of the total weight of the pack, the pack being confined in a retort as described herein. The blades are then subjected to pack difi'usion by placing the retort in an oven maintained at about 900 F. (482 C.) and the blades processed for about 24 hours. Following the pack cementation process, the blades were cooled inside the retort to substantially ambient temperature and the blades then ultrasonically cleaned in water containing about 25 potassium silicate (30.2 Baum).

The alumina is used in the pack as an inert diluent. Besides alumina, other inert and temperature stable diluents can be employed, such as zirconia, titania, hafnia, thoria, rare earth oxides, silicon carbide, titanium carbide, tungsten carbide, and the like. The inert diluent employed is generally refractory in nature and has a melting point above 1300" C.

The magnesium in the pack may range by weight from about 5 to 100%, the refractory diluent up to about by weight, and the halide energizer in small but effective amounts, such as from about A to 5% by weight of the total weight of the pack. The halide energizer may comprise metal and ammoniacal halides and halide formers, such as iodine. Examples of halides are NI-I Cl, NH F, NH I, NH Br and AlCl among others.

In curing the silicate layer, whether applied as a foundation coat or an overcoat, the temperature may range from about 200 F. (93 C.) to 800 F. (426 C.). When applying the silicate layer as a foundation coat prior to pack cementation, the solution, whether it is sodium silicate, potassium silicate or ethyl silicate, may range by weight from about 0.2% to 17.5 SiO equivalent. Where the silicate solution is applied as an overcoat, that is, over the pack cementation coating, the amount of silicate material in the solution may range over the foregoing composition and, more preferably, from about 5% to 17.5 by weight of Si0 equivalent.

As additional examples, the following are given:

EXAMPLE 3 Compressor blades of a titanium alloy containing by weight about 6% Al, 4% V and the balance essentially titanium are honed with finely divided alumina followed by cleaning in an ultrasonic tank containing 10% by volume solution of potassium silicate (produced from 30.2 Baum solution), the temperaure of the solution being about 80 F. (27 C.). The ultrasonic cleaning action removes fine debris and enables a layer of the silicate to form on the blades. The blades are raised out of the solution and allowed to drain and dry by blowing with compressed air. The blades are then sprayed with a solution of 37.5 volume percent potassium silicate (produced from 30.2 Baum solution) which corresponds to about 7.8% by weight of SiO equivalent. The sprayed coating is cured and the step repeated of coating and curing. The treated parts are then placed in a pack mixture containing 40% magnesium powder of 30 mesh (US. Standard) size and 60% alumina, also of 30 mesh size. To the mixture is added 2% by weight of ammonium iodide. Where the pack is being used over again from a prior charge, about a addition is made of magnesium and alumina. After blending the powder, the compressor blades are embedded in the pack and the assembly subjected to pack cementation in a sealed retort at about 750 F. (400 C.) for about 48 hours.

After the blades have been coated, they are removed and cleaned in a hot potassium silicate solution (1% by volume from Baum 30.2 solution) at a temperature ranging from about 140 F. (60 C.) to 160 F. (71 C.).

EXAMPLE 4 An aluminum oxide disc is immersed in a 10% by volume sodium silicate solution (derived from a 41.5 Baum solution) at a temperature of about 80 F. (27 C.), the solution concentration corresponding by weight to 2.9% SiO equivalent. The disc is removed from solution and excess solution allowed to drain off. After drying, the disc is cured at 600 F. (315 C.) for minutes, cooled and again dipped in the solution and dried and cured, until the cycle has been repeated three times.

The thus treated aluminum oxide disc is then embedded in a pack containing magnesium of 100 mesh size (US. Standard) and 80% aluminum oxide of 200 mesh size, 2% iodine being added as the halide-forming transfer agent. The pack which is sealed in a retort is then heated at 850 F. (455 C.) for 24 hours. After the coating has been completed, the disc is removed from the pack and cleaned in a 10 vol. percent sodium silicate solution (based on Baum 41.5).

While the silicate coating is a good source for silicon in producing the magnesium silicide coating by pack cementation, other silicon-containing pre-coats can be employed, such as a fine dispersion of a silica gel. In employing the silicate coating technique, the amount employed is determined in accordance with the percent by weight of SiO equivalent in the coating. Thus, silicate solutions containing about 0.2% to 17.5% of SiO equivalent may be employed in building up a foundation pro-coat preparatory to pack cementation in a magneslum-containing pack.

The markedly improved electro-negative potential and the remarkable oxidation stability of the composite coating make the coating suitable for the protection of low alloy steels, mild steels, titanium, titanium alloys, aluminum and aluminum alloys and ceramics, such as shapes made from alumina.

The invention is particularly applicable to low alloy steels, e.g. AMS 6304, employed in jet or gas engine compressor components operating at over 300 F. (150 C.). The protection of aluminum and aluminum alloys is another very good application. By applying the magnesium coating of the invention to high strength steel or titanium rivets, fasteners, blading and other elements used in direct contact with aluminum and aluminum alloy sheet, castings, and other aluminum shapes or structural elements, the deterioration of the aluminum, as well as the contacting metal, can be greatly inhibited. By employing the invention in situations in which magnesium is in direct contact with steel, the galvanic cell potential developed between these materials can be significantly reduced by producing generalized rather than localized corrosion attack. Thus, as stated above, the invention may be applied to titanium, aluminum, iron, iron alloys, magnesium alloys and numerous ceramic materials, e.g. A1 0 or MgO Where the EMF potential associated with the magnesium-rich coating can 'be desirably utilized. Thus, a direct titanium-aluminum couple can be avoided in aluminum-titanium structural systems by providing one of the metals with the sacrificial coating of the invention by interposin'g the coating between the two metals, such that the magnesium-rich coating corrodes anodically in preference to either of the substrate metals.

As has been stated herein, the cementation pack may range by weight from about 5% to 100% of the magnesium-containing material and up to about by Weight of a refractory diluent, the pack also containing a small but effective amount of a halide energizer. Based on the total weight of the pack, the energizer may range from about A to 5% by weight. A preferred pack is one containing about 30% to 60% of the magnesium material and about 70% to 40% by weight of the diluent, e.g. aluminum oxide.

The cementation process is generally carried out in a retort at a temperature ranging from about 700 F. (about 425 C.) to about 1000 F. (about 538 C.) for about A hour to 60 hours.

As will be apparent from the description, the invention provides as an article of manufacture a thermally coated substrate comprising a reaction product between a siliconcontaining material, e.g. a silicate salt, and magnesium consisting essentially of magnesium, silicon and oxygen, the major portion of the magnesium being in the form of magnesium silicide. Where the substrate is a ferrous metal, a small portion of the coating will contain iron. Likewise, where the substrate is aluminum or titanium, a small amount of such elements may appear in the coating. Generally speaking, the coating will consist essentially of magnesium, silicon and oxygen plus small amounts of substrate material.

Usually, the coating contains at least about 20% by Weight of magnesium, such as about 20% to 50% and, more preferably, 30% to 50% by weight of magnesium, a major portion of which is in the form of magnesium silicide.

As stated hereinbefore, the invention is applicable to the coating of a wide diversity of substrates and, in particular, jet and gas engine turbine components or parts, such as discs, spacers, blades, tie bolts, casings, shrouds, vanes, shafts, and the like.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to Without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

What is claimed is:

1. As an article of manufacture, a metal substrate having a thermally diffusion-bonded coating adhering thereto, the coating being anodically sacrificial relative to said substrate and consisting essentially of magnesium, silicon and oxygen, a major portion of said magnesium in said coating being present as magnesium silicide.

2. As an article of manufacture, a metal gas turbine engine component having a thermally diffusion-bonded coating adhering thereto, said coating being anodically sacrificial relative to said component and consisting essentially of magnesium, silicon and oxygen, a major portion of said magnesium in said coating being present as magnesium silicide.

3. The article of manufacture of claim 2, wherein the component is made of ferrous metal.

4. As an article of manufacture, a metal substrate having a thermally diifusion-bonded coating adhering thereto, said coating being anodically sacrificial relative to said metal substrate and consisting essentially of magnesium, silicon and oxygen, said coating containing at least about 20% by weight of magnesium, a major portion of said magnesium in said coating being present as magnesium silicide.

5. The article of manufacture of claim 4, wherein the substrate is ferrous metal and wherein the coating contains about 20% to 60% magnesium.

nesium.

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